Threaded connection of two metal tubes with high tightening torque

ABSTRACT

A threaded connection for two metal pipes which includes a tapered male thread with trapezoidal threads on a male element and a mating female thread on a female element. The thread width of the male and female threads, at the thread crest, is less than the thread width at the thread root. The width of the thread crests is larger than the width of a space between the roots of the mating threads. The male and female elements are screwed to a position located beyond that of the position where two flanks of the male threads come into contact with two flanks of the female threads. Such a connection permits makeup with a very high torque. The threads of one and/or the other of the threads can optionally include a groove opening into the thread crest.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to threaded connections for two metal pipes with atapered thread and with trapezoidal threads.

Such connections are known, in particular for strings of casing pipes orproduction tubing or drillpipe strings for hydrocarbon wells.

2. Discussion of the Background

In the remainder of the present document, the term “threaded connectionfor two metal pipes” will encompass both an integral connection betweentwo long pipes and a connection between a first, long, pipe and asecond, short, pipe such as a coupling.

The American Petroleum Institute (API) defines:

in specification API 5CT, metal pipes and threaded metal pipeconnections for production and for casing hydrocarbon wells;

and in specification API 5B, standard tapered thread forms for suchconnections.

The threads of such API thread connections can be trapezoidal and thencomprise, on each of the male and female elements, a thread root, athread crest and two flanks, namely a load flank and a stabbing flank.

The thread roots and thread crests are normally parallel to the taper ofthe thread.

The load flanks are so termed because on bearing against each other whenthe connection is subjected to tensile forces, for example due to theweight of the pipes, they enable the connection to tolerate such tensileforces. The load flanks are located on the threads opposite the stabbingflanks.

When making up such an API connection, depending on the taper of thethreads, at a given moment corresponding to a given relative position ofthe male and female elements, the thread roots of one of the elementscome into contact with the thread crests of the other element.

If screwing of the male element is continued into the female elementbeyond that position, the male threads start to interfere radially withthe female elements, which leads to an expansion of the female elementand a contraction in the male element; such an interference must belimited so as not to develop excessive stresses or deformation.

The diametrical interference between the mated points of two surfaces ofrevolution which radially interfere is generally defined as thedifference in the diameter of the half cross section of the surfaces atthose points, the difference being measured before connection and takento be positive when the two surfaces, once connected, exert a contactpressure between the mated points.

To limit such stresses or deformations, an annular bearing surface whichis orientated substantially transverse with respect to the axis of theconnection can be provided on each of the male and female elements, thebearing surfaces being positioned such that they come into abutment witheach other at a given moment during makeup thus precisely defining amakeup completion position.

The position at the end of connection makeup is, for example, determinedby the torque required to arrive at that position.

The use of abutting bearing surfaces to position the connection hasother advantages:

placing the load flanks of the connection threads under tension whichare then ready to tolerate the tensile stresses to which the connectionis subjected during service;

precise positioning of the male and female elements thus guaranteeing,when each of the male and female elements of the connection comprises asealing surface which radially interferes with that located on themating element, a high metal-metal contact pressure between the surfaceswith no risk of plastification thereof;

reduced risk of accidental breakout because of the makeup torque whichhas to be overcome before being able to break out the connection, thistorque being well defined and always above a minimum value.

European patent EP-A-0 488 912 describes such a connection with taperedthreads screwed one into the other, a pair of radially interferingmetal-metal sealing surfaces and a pair of abutting bearing surfaces,namely a concave tapered surface at the end of the male element and aconvex annular surface forming an internal shoulder on the femaleelement.

Such a threaded connection can be made up with high nominal makeuptorques which can be up to 34 kN.m (25000 lbf.ft), for example, which issufficient in the majority of cases.

However, it may be necessary to make up the connection with even highertorques, in particular for casing pipes for multiple deviated wells orhorizontal wells enabling a wide zone to be exploited from a singlesite.

The use of techniques for rotating the string comprising drillpipes attheir end (drilling liner) also permits better cementing of horizontalwells but necessitates pipe connections made up with torques which arehigher than the rotational torque of the string if it is desired toprevent the threaded elements from rotating with respect to each otherwhen the string is rotated, which rotation between elements can modifythe characteristics of use of the connections, in particular theirsealing properties.

Table 1 below gives an idea of the desired makeup torques for suchapplications.

TABLE 1 Desired level of makeup torque External pipe diameter Level ofmakeup torque (mm) (″) (kN · m) (lbf · ft) 101.6-139.7 4″-5″½ 20-3415000-25000 168.3-177.8 6″⅝-7″ 27-41 20000-30000 244.5 9″⅝ 54-8840000-65000

The abutting bearing surfaces can only tolerate such torques withoutdeterioration if the radial width of the abutment surfaces is increased,but then much thicker pipes have to be used which may be incompatiblewith service requirements.

Thus other means have to be used than abutting bearing surfaces toabsorb high makeup torques.

International patent application WO 94/29627 describes a threadedconnection with a tapered thread and trapezoidal threads known as wedgethreads in the general form of a dovetail and more particularly a halfdovetail.

Such threads are known as wedge threads or threads with a variable widthsince the width of the male and female threads varies from one end ofthe thread to the other in a manner which is coordinated between themale and female threads.

Such threads are termed “half dovetail” since they overhang the threadroots on one side only, either on the load flank side, or on thestabbing flank side, and because the angle between the load flank andthe normal to the connection axis and that between the stabbing flankand said normal is such that the thread width is higher at the crestthan at the root.

When the male element is engaged in the female element in accordancewith WO 94/29627, the narrowest thread crests face the widest threadroots and there is a large axial clearance between the mating flanks ofthe threads.

As the male element is screwed into the female element, the axialclearance reduces to a position where the two male flanks come intocontact with their female mates.

Beyond that position, the female flanks interfere with the male flanksand there ensues a very rapid increase in the curve of the makeup torqueas a function of rotation.

Such a connection in accordance with WO 94/25627 can certainly toleratea high makeup torque due to the developed surface of the threads but itsuffers from a number of important disadvantages.

Firstly, variable width wedge threads are expensive to machine anddifficult to inspect.

Further, the acute angles of dovetails or half dovetails disposed on theload flank side and/or on the stabbing flank side, constitute sharpangles which are sensitive to cuts and flash from such cuts aredeleterious to the function of the connection.

Such sharp angles also notch the thread roots and as a result thethreads are more fragile during use.

SUMMARY OF THE INVENTION

The present invention seeks to provide a threaded connection which canbe made up to a high makeup torque T which is free of such disadvantagesand in particular a threaded connection which is economical to machineand which can be readily manipulated on-site.

We have also sought to provide a threaded connection whereby the desiredmakeup torque is obtained after considerable rotation, for example ofthe order of one turn, or more.

We have also ensured that in certain configurations, the slope of themakeup torque—rotation curve is reduced from a given torque, resultingin a self-limiting characteristic for the makeup torque.

We have also sought to provide a connection which is particularly tightto internal and/or external fluids, even after a number ofmakeup-breakout operations.

The threaded connection between two metal pipes of the present inventioncomprises a male element at the end of a first pipe screwed into afemale element at the end of a second pipe.

The male element has an external tapered male thread with trapezoidalthreads where the thread width at the thread crests is less than thethread width at the thread root.

The female element comprises an internal tapered female thread withtrapezoidal threads with a form which mates with that of the malethread.

The term “female thread mating with that of the thread” here means thatthe taper and pitch of the female thread are substantially identical tothose of the male thread and that the thread form of the female threadsis substantially identical to that of the male threads, the inclinationof the load flanks and stabbing flanks of the female threads to theconnection axis being in particular identical to that of thecorresponding flanks on the male elements, the width of the femalethread crests being less than that of their root as with the malethreads. Clearly, the form of the male thread then reciprocally mateswith that of the female thread.

The width of the thread crests on each of the male and female threads ishigher than the width of the space between the roots of the matingthreads.

The male element is positioned by screwing into the female element to arelative position of these two elements located beyond that where,during makeup, the two male thread flanks come into contact with the twofemale thread flanks, so as to induce an axial interference fit of themale threads by the female threads, and vice versa.

Depending on the mating form of the male and female trapezoidal threadsused and because the thread width at the crest is lower than the threadwidth at the root, the male and female threads penetrate radially andwedge into the mating hollows by a wedging effect as the axialprogression of the threads occurs during makeup and thus, beyond theposition of contact of the two mating flanks, induce an axialinterference fit of the male threads by the female threads and viceversa.

This interference fit over all of the surface of the flanks results inthe possibility of absorbing a high degree of makeup torque T in thethreads.

The features of such threads are such that they can be made cheaply,they are easy to inspect and are not fragile in use.

EP-A-0 454 147 describes a threaded connection with a tapered thread andtrapezoidal threads where the width of the female thread crests ishigher than that of the male thread roots and in which when connectionmakeup is complete the two flanks of the thread of one element are incontact with those of the mating element, at least over a portion of thethread.

However, in EP-A-0 454 147, only simple contact has been aimed at, evenonly partial contact, between the mating flanks so that, when theconnection is to be subjected to compression stresses after having beensubjected to tensile stresses, there is no re-positioning of thestabbing flanks due to an axial clearance which pre-exists with thelatter, which re-positioning can cause plastification of the metal, inparticular the sealing surfaces, and can thus cause a subsequent risk ofleakage when the connection is again subjected to tensile stresses. Toobtain such a simple contact of the thread flanks when connection makeupis complete, EP-A-454 147 has to use means for positioning the elements,namely a transverse bearing surface on each element, each bearingsurface being able to abut against that of the mating element. Themating male and female stabbing flanks are disposed so as to provide aminimum axial clearance between them before the bearing surfaces comeinto contact, which axial clearance reduces to zero or almost zero whenthe bearing surfaces come into abutment. These transverse abuttingsurfaces can also, conventionally, absorb the makeup torque, but EP-A-0454 147 does not disclose a high makeup torque threaded connection.

In the present invention, when connection makeup is complete, thediametrical interference between the thread crests of each of the twomale and female threads and the mating thread roots is preferablynegative or zero.

Very preferably, when the two flanks of the male thread come intocontact with their mating female threads during makeup, a radialclearance of at least 0.15 mm subsists between the crests and roots ofthe mated threads.

In a preferred variation, the diametrical interference between thethread crests of one only of the two male or female threads and thethread roots of the mated thread is positive when makeup is complete.

Preferably, to obtain an effective axial interference fit of the threadsby a wedge effect, the angle δ between the load flank and the stabbingflank of the male or female threads is less than or equal to 20°.

Highly preferably, it is in the range 7° to 20° and more particularly,close to 10°.

Preferably again, the thread crests of each of the male and femalethreads overhang the thread roots of the same thread on the load flankside, the angle α between the load flank and the normal to theconnection axis thus being negative and having a value in the range 0 to−15°.

Advantageously, if too severe a wedge effect causing too high a slopedT/dN of the curve of the makeup torque as a function of the number ofturns is to be attenuated, at least one of the male and/or femalethreads comprises a groove opening into the thread crest over all or aportion of the length of the thread or threads.

Such a groove increases the flexibility of the thread and somewhatreduces the axial interference fit forces and reduces the more theseforces as the groove has a substantial depth and width. This results ina substantial reduction in the slope dT/dN of the curve of the makeuptorque as a function of the number of turns at the expense of a slightreduction in the maximum makeup torque. Thus the combination of the twocharacteristics, maximum makeup torque and slope dT/dN, can beoptimised.

French patent FR-A-2 408 061 describes threaded connections withtrapezoidal threads in which one of the threads carries a type of grooveopening into the thread crest.

However, that groove is closely associated with structures of the threadflanks producing a self-locking connection, i.e., resisting breakout: tothis end, the inclination of the thread flanks with the groove isdifferent from that of the thread flanks with no groove and is such thatthe width of the groove at its opening reduces during makeup under thebending forces resulting from the differences in orientation of themating thread flanks.

This document does not disclose the function of the means of theconnection of the invention and is not applicable to threads with thecharacteristics of the invention, in particular a female thread formwhich mates with the male threads.

Preferably, in accordance with the invention, the groove depth is atmost equal to the thread depth and the groove width at its opening intothe thread crest is at most ⅔ of the thread width, the thread depthbeing the radial distance measured perpendicular to the connection axisbetween the taper enveloping the thread crests and that of the threadroots and the thread width being measured parallel to the connectionaxis at the thread mid-depth.

Preferably again, the groove has, in a longitudinal axial plane, a Uprofile with arms which may or may not be parallel, or in the shape of aV with a rounded base.

Very preferably, the rounded base of the groove has a radius of at least0.2 mm to prevent stress concentration in the groove base.

Preferably again, each of the male and female elements comprises atleast one sealing surface, the orientation of each male sealing surfacebeing substantially longitudinal and radially interfering with a matingfemale sealing surface at the end of the connection makeup so as to sealthe connection.

Preferably again, each of the male and female elements comprises atleast one substantially transverse bearing surface, at least oneabutting male bearing surface coming into abutment against a femalebearing surface at the end of the connection makeup to preciselyposition the sealing surfaces and thus define their interference.

Such bearing surfaces do not, however, act to define the position at theend of connection makeup when the stabbing flanks of the threads comeinto contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures illustrate particular non limiting embodiments ofthe invention.

FIG. 1 shows an axial half cross section of one end of a pipe orcoupling comprising a female element of a threaded connection of theinvention.

FIG. 2 shows an axial half cross section of one end of a further pipecomprising a male element of a threaded connection of the invention.

FIG. 3 shows an axial half cross section of a threaded connectionobtained after makeup of the elements of FIGS. 1 and 2.

FIG. 4 is a schematic half cross section of a detail of a few femalethreads of the female element of FIG. 1.

FIG. 5 is a schematic half cross section of a detail of a few malethreads of the male element of FIG. 2.

FIG. 6 is an axial half cross section of a detail of a few threads FIGS.4 and 5 during makeup, the connection being of the type shown in FIG. 3.

FIGS. 7 and 8 show a variation of FIGS. 4 and 5.

FIG. 9 is a schematic half cross-section of a detail of a few threads ofFIGS. 7 and 8 when connected.

FIG. 10 is a graph of the makeup torque T as a function of the number ofturns N in two variations of the connection of FIG. 6.

FIG. 11 is a variation of FIG. 4 with a groove in the female threads.

FIG. 12 is a variation of FIG. 5 with a groove in the male threads.

FIG. 13 shows a graph of the makeup torque T as a function of the numberof turns N in two variations of the connection of elements of FIGS. 11and 12.

FIG. 14 shows the application of the present invention to a threaded andcoupled connection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a threaded connection 100 between a male element 1 at oneend of a first metal pipe 101 and a female element 2 at one end of asecond metal pipe 102 which may be a long pipe or a coupling. Suchthreaded connections can, for example, constitute strings of casing pipeor production tubing for hydrocarbon wells.

Male element 1 shown in FIG. 2 comprises on its external surface atapered male thread 3 with trapezoidal threads and its end, which isalso the end of the first pipe, has an annular transverse male endsurface 7.

Female element 2 shown in FIG. 1 has on its inter surface a taperedfemale thread 4 which mates with male thread 3.

Pipes 101 and 102 are connected by screwing male thread 3 on maleelement 1 into female thread 4 on female element 2.

The connection of FIG. 3 optionally comprises, on each of the elements,additional means when the connection needs to be particularly tight,namely:

a) on the male element, an external tapered male sealing surface 5 thetaper of which is generally higher that that of the male thread 3; thetaper of the male sealing surface 5 measured on the diameter is 20%, forexample;

b) on the female element:

an internal tapered female sealing surface 6 the taper of which issubstantially identical to that of the male sealing surface 5;

an internal shoulder with an annular and transverse bearing surface 8.

The male end surface 7 can, in known manner, be concave tapered with avery open vertex half angle, for example 75°, the female bearing surface8 in this case being convex with the same vertex half angle.

The additional means 5, 6, 8, which are optional as regards theinvention, function as follows on the connection 100.

The male sealing surface 5 interferes radially with the female sealingsurface 6, i.e., its diameter at a reference point before connection ishigher than the diameter of the mating point on the female sealingsurface 6, this diameter also being measured before connection.

During makeup, once the sealing surfaces have made contact, continuingmakeup induces an increasing diametrical interference in the sealingsurfaces.

The precise position on makeup completion is determined by abutment ofthe male end surface 7 against bearing surface 8 of the internal femaleshoulder, which defines a precise value for the interference between thesealing surfaces 5, 6.

The position when connection is complete can be marked by a given valueof the makeup torque.

The concave-convex form of bearing surfaces 7, 8 prevents the bearingsurfaces from coming apart and increases the contact pressure betweenthe sealing surfaces.

FIGS. 4, 5 and 6 show features of the male and female threads of aconnection of the invention.

Male thread 3 (FIG. 5) is a tapered thread with a pitch P the pitchsurface directrix line 21 (abbreviated to pitch line) of which isinclined at an angle γ to the connection axis such that γ=arctan(TT/200), TT being the taper of the thread with respect to the diameterand expressed as a %.

The male threads 11 are trapezoidal threads with a thread crest 17, twoflanks 13 and 15 and are separated by a thread root 19.

The thread crests 17 and thread roots 19 are parallel to the male pitchline 21.

The flanks comprise a load flank 13 and a stabbing flank 15, the latterbeing turned towards the male end surface 7.

The load flanks 13 and stabbing flanks 15 are respectively inclined atan angle α and β with respect to the normal to the axis of theconnection with an angle δ between them.

The thread crests 17 overhang the thread roots 19 on the side of theload flanks 13 in FIG. 5 so as to prevent the threads from jumping outduring makeup. By convention, the angle α is negative, the angle β onthe non overhanging side being positive.

The angle is also the algebraic sum of (α+β) and its apex is directedtowards the exterior of the male thread such that the male threads 11have a width l1 at the root which is higher than that l3 at their crest.

The female thread 4 (FIG. 4) has matching characteristics to those ofmale thread 3.

The female pitch line 22 of the female thread is inclined at an angle γto the connection axis X, which angle is identical to the inclination ofthe male pitch line 21.

The crests 20 and roots 18 of the female thread are parallel to thefemale pitch line 22.

The load flanks 14 and stabbing flanks 16 of the female thread arerespectively inclined at an angle α and β to the normal to theconnection axis X and form an angle δ between them, each of these anglesbeing identical to the corresponding male angle.

The width l3 of the male thread crest 17 is slightly larger than thewidth l6 of the female thread root 18, for example by 0.2 mm.

The width l4 of the female thread crest 20 is slightly larger than thewidth l5 of the male thread root 19, for example by 0.2 mm.

When the male thread 3 is screwed into the female thread 4, at a givenmoment illustrated in FIG. 6, the two flanks 13, 15 of male threads 11come into contact with the two flanks 14, 16 of female threads 12 oversubstantially the entire depth of these flanks.

This results from the fact that the width of the male and female crests3, 4 is larger than the width of the space l6, l5 between the roots ofthe mating threads, and from the trapezoidal form of the threads, withl1>l3 and l2>l4.

If makeup is continued, male threads 11 penetrate radially and wedgeinto the female hollows and similarly, female threads 12 bury radiallyand wedge into the male hollows.

For this reason, there exists an axial interference fit of the malethread flanks and female thread flanks and it is necessary toconsiderably increase the torque to continue makeup. This results in avery steep slope dT/dN of the curve of the makeup torque as a functionof the number of turns N, depending on the surface area of theinterference fit flanks which are in contact over substantially theentire depth and over their length.

The slope dT/dN is a function of the modulus of elasticity of the metalof the elements, of the taper TT of the thread, of the length of thethread, the mean diameter thereof, the thread depth, the angle δ betweenthe flanks of a thread and of the coefficient of friction between themale and female threads. It is thus possible to predict the slope dT/dN.

In order for the male and female threads to be able to penetrateradially into the mating hollows, angle δ of the wedge formed by threadflanks 11, 12 is advantageously not too large and remains, for example,below 20°.

An angle δ which is too low is also not desirable as the wedgephenomenon will be more difficult to implement: preferably, angle δ isin the range [7°, 20°], with an angle δ of 10° being preferred.

The choice of threads with load flanks 13, 14 having a negative angle α,for example α=−3° can also enable an angle β of +13° to be used which issufficiently inclined to enable the threads to engage easily.

The maximum admissible value of the makeup torque T is determined by theelastic limit of the metal of the elements. It is thus possible topredict the maximum admissible value for the makeup torque.

In order to obtain an optimum wedge effect without excessive stresses inthe male and female elements and to allow the grease disposed on thethreads before connection to flow, any radial interference between thethread crests (17, 20) of each of the two male and female threads (3, 4)and the thread roots (18, 19) of the mated thread (4, 3) should beavoided; thus threads can be designed, in particular in their width, inwhich the diametrical interference between the thread crests of each oftwo male and female threads and the thread roots of the mated thread isnegative or zero when makeup is complete.

In particular, threads can be designed for which at the moment of firstcontact on the two flanks (FIG. 6), the value x1 of the radial clearancebetween the male thread crest 17 and the female thread hollow 18 andthat x2 of the radial clearance between the female thread crest 20 andthe male thread hollow 19 is at least 0.15 mm.

Instead of avoiding any radial interference between the thread crests ofeach of the two male and female threads (3, 4) and the thread roots ofthe mated thread (4, 3), in a variation, threads 71, 72, in particulartheir width and depth, can be designed to create a positive diametricalinterference between the thread crests of one of the male or femalethreads and the thread roots of the mating thread when makeup iscomplete.

Such a variation is shown in FIGS. 7, 8 and 9 respectively for thefemale thread 4, the male thread 3 and the connection of the two; whenmakeup is complete (FIG. 9), a clearance x exists between the malethread crests 77 and the female thread roots 78 while the female threadcrests 80 radially interfere with the male thread roots 79. Such adisposition has a relatively small influence on the level of the makeuptorque.

Preferably for this variation, contact between the female thread crests80 and the male thread roots 79 occurs before contact between the matingflanks 73, 74, 75, 76.

FIG. 10 shows a graph of the makeup torque T as a function of the numberof turns N for the following geometry for pipes of a connection as shownin FIG. 6:

pipe diameter: 177.80 mm

pipe thickness: 10.36 mm (29 lb/ft);

pipe material: low alloy steel with yield stress ≧551 MPa;

5 threads per inch thread;

thread taper TT=6.25% on the diameter (γ=1.79°);

angle α of load flank=−3°;

angle β of stabbing flank: +9° and +13°;

angle δ between flanks: 6° and 10°.

On this graph, the abscissa N=0 corresponds to the first contact on thetwo flanks 13-14 and 15-16 of mated threads.

For threads with β=+90° (δ=6°), a torque of more than 35 kN.m (26000lbf.ft) was obtained in 0.8 of a revolution, corresponding to a slope of44.5 kN.m/turn.

For threads with β=+13° (δ=10°), a torque of 45.9 kN.m (33850 lbf.ft)was obtained in 0.47 of a revolution, corresponding to a slope of 97.5kN.m/turn.

Such a thread with interference fit thread flanks enables male andfemale elements to be made up with a very high torque without the needfor an abutment but such a torque is then obtained by a rotation of afraction of a turn of the male and female elements, which may prove tobe insufficient particularly when sealing surfaces are to be provided onthe male and female elements such as, for example, 5, 6 in FIG. 3.

The development of a given contact pressure between such sealingsurfaces necessitates a rotation of one element with respect to theother of the order of one turn after the sealing surfaces come intocontact and a precise positioning of the male and female elements whenmakeup is complete, the positioning being obtained using an abutmentconstituted by two transverse bearing surfaces such as 7, 8 in FIG. 3.

It is difficult to guarantee satisfactory sealing during service on athreaded connection with interference fit thread flanks of the type ofFIGS. 4, 5, 6, the sealing surfaces and the transverse abutment bearingsurfaces of the type shown in FIG. 3, taking into account themanufacturing tolerances appropriate for the male and female elements.

A reduction of the angle δ between the thread flanks would certainlyreduce the slope of the curve of the makeup torque as a function of thenumber of turns but, unless a highly negative angle α of less than −15°is used, which is not desirable, it would be necessary to substantiallyreduce the angle β of the stabbing flanks, which is also not desirable.

The thread forms illustrated in FIGS. 11 and 12 can resolve the problemsof compatibility between the high values of the nominal makeup torqueand not too high a value, of the order of 10 kN.m/turn, for example, ofthe slope dT/dN of the curve of the torque as a function of the numberof turns.

The threads of FIGS. 11 and 12 are particularly advantageous in the caseof a connection with sealing surfaces and an abutment of the type shownin FIG. 3.

The male threads of FIG. 12 have, as in FIG. 5, a load flank 33 inclinedat a negative angle α with respect to the normal to the connection axis,a stabbing flank 35 inclined at a positive angle β with respect to thissame normal, a thread crest 39 and are separated by a thread root 37.

The features of female threads 32 of FIG. 11 correspond to those of malethreads 31 in FIG. 12 with a load flank 34, a stabbing flank 36, athread crest 38 and a thread root 40 with the same pitch and orientationas those of the male threads 31 with respect to the connection axis.

The width of the male and female thread crests 39, 38 is higher thanthat of the mating thread roots 40, 37 such that at a given momentduring makeup, the male flanks 33, 35 are both in contact with theirfemale mating threads 34, 36 although a radial clearance subsistsbetween the thread crests 39, 38 and the mating thread roots 40, 37. Ina variation which is not shown, there could be a positive radialinterference between the thread crest of a single thread and the matingthread root.

Male thread 31 has a groove 61 opening into the male thread crest 39,the profile of the groove being a V with a rounded base and in which theaxis of the V is substantially normal to the connection axis.

The depth of the groove 61, measured radially, is 65% of the depth ofmale thread 31 and its base is a circular arc with radius 0.4 mm.

The angle between the two arms of the V of groove 61 is 35° and inducesin the male thread 31 of FIG. 12 a groove width, at its opening into thethread crest, of 34% of the width of the male thread measured atmid-depth.

Female thread 32 has a groove 62 opening into the crest of female thread38, with the same geometry as groove 61 in male thread 31 and disposedin the female thread in the same manner as groove 61.

Such grooves 61, 62 transform each of threads 31, 32 subjected tocompression stresses on the two flanks 33, 34, 35, 36 into twocantilevers 63+65, 64+66, subjected to bending stresses.

The resultant flexibility induces a reduction in the interference fitforces when the connection is made up beyond the point of simultaneouscontact of the flanks and as a result reduces the slope dT/dN of thecurve of the makeup torque as a function of the rotation.

The groove depth and its width at its opening into the thread crest areparameters which can be acted upon to obtain a slope dT/dN of the curveof the makeup torque as a function of the rotation which is not toohigh, which remains, for example, of the order of 20 kN.m/turn.

The depth of the groove must be less than or equal to the thread depthas otherwise it could unacceptably weaken the structure of theconnection.

The groove width at its opening into the thread crest must be less thanor equal to ⅔ of the width of the thread measured at mid height thereofto preserve a sufficient rigidity at the level of the cantilevers 63,64, 65, 66.

The 0.4 mm radius for the groove base limits stress concentration at thegroove base.

Curve E in FIG. 13 shows such a thread structure with a groove allowingmakeup with a nominal torque of over a minimum desired value for thepipe dimension under consideration (27 kN.m: see Table 1). The slopedT/dN of the graph of the makeup torque changes beyond 25 kN.m: theslope is of the order of 24 kN/turn at the start and it reduces above 25kN.m to about 9 kN.m.turn, which self-limits the makeup torque andprecisely positions the abutments and sealing surfaces. By way ofcomparison, curve D in FIG. 13 relates to similar threads but without agroove, and has a relatively constant slope of the order of 40 kN/turn.

The connection of the present invention can be produced in a number ofvariations; the embodiments described here are non limiting.

In particular, the present invention can be applied both to:

an integral threaded connection 100, a male element 1 being disposed atthe end of a first long metal pipe 101 and a female element 2 beingdisposed at the end of a second long pipe 102;

and to a threaded and coupled connection 200 shown in FIG. 14, in whichtwo long metal pipes 101, 101′ comprising a male element 1, 1′ at theend are connected via a metal coupling 202 which is provided at each ofits ends with a female element 2, 2′, such a threaded and coupledconnection using two threaded connections 100, 100′ of the invention.

What is claimed is:
 1. A threaded connection for two metal pipes,comprising a male element at the end of a first metal pipe screwed intoa female element at the end of a second metal pipe, the male elementcomprising an external male tapered thread with trapezoidal threadscomprising two flanks, namely a load flank and a stabbing flank, athread width at a male thread crest being less than the thread width atthe root of the male thread, the female element comprising an internaltapered female thread with trapezoidal threads with a form which mateswith the male thread, the width of the thread crests on each of the maleand female threads being higher than the width of a space between theroots of the mating threads, characterized in that the male element ispositioned by screwing into the female element to a relative position ofthese two elements located beyond the relative position where, duringmakeup, the two flanks of the male threads come into contact with thetwo flanks of the female threads so as to induce an axial interferencefit of the male threads by the female threads and vice versa.
 2. Athreaded connection according to claim 1, characterized in that thediametrical interference between the thread crests of each of the twomale and female threads and the thread roots of the mating thread isnegative or zero when makeup is complete.
 3. A threaded connectionaccording to claim 1, characterized in that an axial clearance of atleast 0.15 mm subsists between the mated thread crests and roots whenthe two flanks of the male threads come into contact with their femalemates during makeup.
 4. A threaded connection according to claim 1,characterized in that the diametrical interference between the threadcrests of one of the two male or female threads and the thread roots ofthe mating thread is positive at the end of connection makeup.
 5. Athreaded connection according to claim 1, characterized in that theangle (δ) between the load flank and the stabbing flank of the male orfemale threads is 20° or less.
 6. A threaded connection according toclaim 5, characterized in that the angle (δ) is in the range 7° to 20°.7. A threaded connection according to claim 1, characterized in that foreach of the male and female threads the thread crests overhang thethread roots on the load flank side, an angle between said load flanksand the normal to the connection axis being in the range 0 to −15°.
 8. Athreaded connection according to claim 1, characterized in that at leastone of the male and female threads comprises a groove opening into thethread crest over all or a portion of the thread length.
 9. A threadedconnection according to claim 8, characterized in that the depth of thegroove is at most equal to the thread depth.
 10. A threaded connectionaccording to claim 8, characterized in that the width of the groove atits opening into the thread crest is at most ⅔ of the width of thethread measured at the thread mid-depth.
 11. A threaded connectionaccording to claim 8, characterized in that the groove has a profile inthe shape of a U or a profile in the shape of a V with a rounded base.12. A threaded connection according to claim 1, characterized in thateach of the male and female elements comprise at least one sealingsurface, each male sealing surface having a substantially longitudinalorientation and radially interfering with a mating female sealingsurface when makeup is complete.
 13. A threaded connection according toclaim 1, characterized in that each of the male and female elementscomprise at least one bearing surface with a substantially transverseorientation, at least one male bearing surface being abutted with afemale bearing surface when makeup is complete.
 14. A threadedconnection according to claim 1, wherein the first metal pipe and thesecond metal pipe are elongated in length.
 15. Use of two threadedconnections according to claim 1 to connect two long metal pipes whichcomprise a male element at their end via a metal coupling which isprovided with a female element at each of its ends.